The response of lake margin sedimentary systems to climatically driven lake level fluctuations: Middle Devonian, Orcadian Basin, Scotland

Lake margin sedimentary systems can provide highly sensitive records of sedimentary response to climate change. The Middle Old Red Sandstone of Northern Scotland comprises a thick succession of cyclic lacustrine sediments. Within this succession the deepest lake phase, the Achanarras fish bed, allows bed-scale correlation over 160 km across the basin. This provides a unique opportunity to examine the character of synchronous lake margin deposits, and their response to climatically driven lake level fluctuations, across a large continental basin. Detailed characterisation of two separate lake margin systems was carried out utilising multiple sections in western Orkney, in the north, and Easter Ross, in the south. Seven facies have been recognised, which include upper and lower shoreface, deep lake, shallow lake, playa, turbidite and fluvial facies. Differences in vertical and lateral facies stacking patterns reflect the response of these systems to climatically driven fluctuations in lake level. Comparison of the northern and southern systems examined highlights the variable response of lake margin systems to the same climatic change and related lake level fluctuations. In the south, a greater fluvial influence is recognised on the development of the lake margin successions, whereas in the northern example, which lay on the downwind margin of the lake, shore zone facies are more commonly developed. The variability recognised can be accounted for by regional variations in sediment supply, coastal physiography, lake size, bathymetry and potential fetch. Lake level stability is also recognised as a major control on the development of lake margin sedimentary systems, as is the linked or unlinked relationship of the catchment and the lake basin climate for which a conceptual model is proposed.